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Re: [Xen-devel] [v3,11/41] mips: reuse asm-generic/barrier.h

On 01/14/2016 12:48 PM, Paul E. McKenney wrote:

So SYNC_RMB is intended to implement smp_rmb(), correct?

You could use SYNC_ACQUIRE() to implement read_barrier_depends() and
smp_read_barrier_depends(), but SYNC_RMB probably does not suffice.

If smp_read_barrier_depends() is used to separate not only two reads but read pointer and WRITE basing on that pointer (example below) - yes. I just doesn't see any example of this in famous Documentation/memory-barriers.txt and had no chance to know what you use it in this way too.

The reason for this is that smp_read_barrier_depends() must order the
pointer load against any subsequent read or write through a dereference
of that pointer.

I can't see that requirement anywhere in Documents directory. I mean - the words "write through a dereference of that pointer" or similar for smp_read_barrier_depends.

   For example:

        p = READ_ONCE(gp);
        r1 = p->a; /* ordered by smp_rmb(). */
        p->b = 42; /* NOT ordered by smp_rmb(), BUG!!! */
        r2 = x; /* ordered by smp_rmb(), but doesn't need to be. */

In contrast:

        p = READ_ONCE(gp);
        r1 = p->a; /* ordered by smp_read_barrier_depends(). */
        p->b = 42; /* ordered by smp_read_barrier_depends(). */
        r2 = x; /* not ordered by smp_read_barrier_depends(), which is OK. */

Again, if your hardware maintains local ordering for address
and data dependencies, you can have read_barrier_depends() and
smp_read_barrier_depends() be no-ops like they are for most

It is not so simple, I mean "local ordering for address and data dependencies". Local ordering is NOT enough. It happens that current MIPS R6 doesn't require in your example smp_read_barrier_depends() but in discussion it comes out that it may not. Because without smp_read_barrier_depends() your example can be a part of Will's WRC+addr+addr and we found some design which easily can bump into this test. And that design actually performs "local ordering for address and data dependencies" too.

- Leonid.

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